Superseded by a more recent version INTERNATIONAL TELECOMMUNICATION UNION GENERAL ASPECTS OF DIGITAL TRANSMISSION SYSTEMS

Transcription

1 INTERNATIONAL TELECOMMUNICATION UNION ITU-T G.74 TELECOMMUNICATION (7/95) STANDARDIZATION SECTOR OF ITU GENERAL ASPECTS OF DIGITAL TRANSMISSION SYSTEMS SYNCHRONOUS FRAME STRUCTURES USED AT 544, 632, 248, 8488 AND kbit/s HIERARCHICAL LEVELS ITU-T Recommendation G.74 Superseded by a more recent version (Previously CCITT Recommendation )

2 FOREWORD The ITU-T (Telecommunication Standardization Sector) is a permanent organ of the International Telecommunication Union (ITU). The ITU-T is responsible for studying technical, operating and tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide basis. The World Telecommunication Standardization Conference (WTSC), which meets every four years, establishes the topics for study by the ITU-T Study Groups which, in their turn, produce Recommendations on these topics. The approval of Recommendations by the Members of the ITU-T is covered by the procedure laid down in WTSC Resolution No. (Helsinki, March -2, 993). ITU-T Recommendation G.74 was revised by ITU-T Study Group 5 ( ) and was approved under the WTSC Resolution No. procedure on the th of July 995. NOTE In this Recommendation, the expression Administration is used for conciseness to indicate both a telecommunication administration and a recognized operating agency. ITU 99 All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from the ITU.

3 CONTENTS Page Scope... 2 Basic frame structures Basic frame structure at 544 kbit/s Basic frame structure at 632 kbit/s Basic frame structure at 248 kbit/s Basic frame structure at 8448 kbit/s Basic frame structure at kbit/s... 3 Characteristics of frame structure carrying channels at various bit rates in 544 kbit/s Interface at 544 kbit/s carrying 64 kbit/s channels Interface at 544 kbit/s carrying 32 kbit/s channel time slots Interface at 544 kbit/s carrying n 64 kbit/s Characteristics of frame structures carrying channels at various bit rates in 632 kbit/s interfaces Interface at 632 kbit/s carrying 64 kbit/s channels Interfaces at 632 kbit/s carrying other channels than 64 kbit/s Characteristics of frame structures carrying channels at various bit rates in 248 kbit/s interfaces Interface at 248 kbit/s carrying 64 kbit/s channels Interface at 248 kbit/s carrying n 64 kbit/s Characteristics of frame structures carrying channels at various bit rates in 8448 kbit/s interface Interface at 8448 kbit/s carrying 64 kbit/s channels Interface at 8448 kbit/s carrying other channels than 64 kbit/s... 3 Annex A Examples of CRC implementations using shift registers A. CRC-6 procedure for interface at 544 kbit/s A.2 CRC-5 procedure for interface at 632 kbit/s A.3 CRC-4 procedure for interface at 248kbit/s Annex B Alphabetical list of abbreviations used in this Recommendation Recommendation G.74 (7/95) Superseded by a more recent version i

4 SUMMARY This Recommendation gives functional characteristics of interfaces associated with: network nodes, in particular, synchronous digital multiplex equipment and digital exchanges in IDNs for telephony and ISDNs; and PCM multiplexing equipment. Clause 2 deals with basic frame structures, including details of frame length, frame alignment signals, Cyclic Redundancy Check (CRC) procedures and other basic information. Clauses 3 to 6 contain more specific information about how certain channels at 64 kbit/s and at other bit rates are accommodated within the basic frame structures described in 2. ii Recommendation G.74 (7/95) Superseded by a more recent version

5 Recommendation G.74 Recommendation G.74 (7/95) Superseded by a more recent version Superseded by a more recent version SYNCHRONOUS FRAME STRUCTURES USED AT 544, 632, 248, 8488 AND kbit/s HIERARCHICAL LEVELS (Malaga-Torremolinos, 984; amended at Melbourne, 988; revised in 99 and 995) Scope This Recommendation gives functional characteristics of interfaces associated with: network nodes, in particular, synchronous digital multiplex equipment and digital exchanges in IDNs for telephony and ISDNs; and PCM multiplexing equipment. Clause 2 deals with basic frame structures, including details of frame length, frame alignment signals, Cyclic Redundancy Check (CRC) procedures and other basic information. Clauses 3 to 6 contain more specific information about how certain channels at 64 kbit/s and at other bit rates are accommodated within the basic frame structures described in 2. Electrical characteristics for these interfaces are defined in Recommendation G.73. NOTES This Recommendation does not necessarily apply to those cases where the signals that cross the interfaces are devoted to non-switched connections, such as those for the transport of encoded wideband signals (e.g. broadcast TV signals or multiplexed sound-program signals which need not be individually routed via the ISDN), see also Annex A/G The frame structures recommended in this Recommendation do not apply to certain maintenance signals, such as the all s signals transmitted during fault conditions or other signals transmitted during out-of-service conditions. 3 Frame structures associated with digital multiplexing equipments using justification are covered in each corresponding equipment Recommendation. 4 Inclusion of channel structures at other bit rates than 64 kbit/s is a matter for further study. Recommendations G.76 and G.763 dealing with the characteristics of PCM/ADPCM transcoding equipment contain information about channel structures at 32 kbit/s. The more general use of those particular structures is a subject of further study. 2 Basic frame structures 2. Basic frame structure at 544 kbit/s 2.. Frame length 93 bits, numbered to 93. The frame repetition rate is 8 Hz F-bit The first bit of a frame is designated an F-bit, and is used for such purposes as frame alignment, performance monitoring and providing a data link Allocation of F-bit Two alternative methods as given in Tables and 3 for allocation of F-bits are recommended Method : Twenty-four-frame multiframe Allocation of the F-bit to the multiframe alignment signal, the CRC check bits and the data link is given in Table. Recommendation G.74 (7/95) Superseded by a more recent version

6 TABLE /G.74 Multiframe structure for the 24 frame multiframe F-bit Frame number within multiframe Bit number within multiframe Assignments FAS DL CRC Bit number(s) in each channel time slot For character signal a) Signalling channel designation For a) signalling a) m e m m e A 7 59 m m e m B m e m m e C m m e m D FAS Frame alignment signal (......). DL 4 kbit/s data link (message bits m). CRC CRC-6 block check field (check bits e to e 6 ). a) Only applicable in the case of channel associated signalling (see ) Multiframe alignment signal The F-bit of every fourth frame forms the pattern.... This multiframe alignment signal is used to identify where each particular frame is located within the multiframe in order to extract the cyclic redundancy check code, CRC-6, and the data link information, as well as to identify those frames that contain signalling (frames 6, 2, 8 and 24), if channel associated signalling is used Cyclic Redundancy Check (CRC) The CRC-6 is a method of performance monitoring that is contained within the F-bit position of frames 2, 6,, 4 8 and 22 of every multiframe (see Table ). 2 Recommendation G.74 (7/95) Superseded by a more recent version

7 The CRC-6 message block check bits e, e 2, e 3, e 4, e 5, and e 6 are contained within multiframe bits 94, 966, 738, 25, 3282 and 454 respectively, as shown in Table. The CRC-6 Message Block (CMB) is a sequence of 4632 serial bits that is coincident with a multiframe. By definition, CMB N begins at bit position of multiframe N and ends at bit position 4632 of multiframe N. The first transmitted CRC bit of a multiframe is the most significant bit of the CMB polynomial. In calculating the CRC-6 bits, the F-bits are replaced by binary s. All information in the other bit positions will be identical to the information in the corresponding multiframe bit positions. The check-bit sequence e through e 6 transmitted in multiframe N +, is the remainder after multiplication by x 6 and then division (modulo 2) by the generator polynomial x 6 + x + of the polynomial corresponding to CMB N. The first check bit (e ) is the most significant bit of the remainder; the last check bit (e 6 ) is the least significant bit of the remainder. Each multiframe contains the CRC-6 check bits generated for the preceding CMB. At the receiver, the received CMB, with each F-bit having first been replaced by a binary, is acted upon by the multiplication/division process described above. The resulting remainder is compared on a bit-by-bit basis, with the CRC-6 check bits contained in the subsequently received multiframe. The compared check bits will be identical in the absence of transmission errors kbit/s data link Beginning with frame of the multiframe (see Table ), the first bit of every other frame is part of the 4 kbit/s data link. This data link provides a communication path between primary hierarchical level terminals. In prioritized order the data link will contain priority operations messages, other maintenance or operations messages, periodic terminal performance reports, or an idle data link sequence. Both categories of operations messages are transmitted in the form of 6-bit sequences of the form P P 2 P 3 P 4 P 5 P 6, where the particular message is coded by the six bits P through P 6, permitting up to 64 distinct messages. Designated sequences and their functional uses are listed in Table 2. Use of sequences not shown in the table is for further study. Periodic terminal performance reports are formatted using the unnumbered, unacknowledged frame option of Q.92/LAPD, as described in Transmission of any of the assigned 6-bit sequences terminates any processing of a (lower priority) performance report that might be in progress, as seven or more consecutive ones are recognized in Recommendation Q.92 as an abort command. The idle data link sequence is a continuous repetition of the pattern. The transmission of Loss of Frame Alignment (LFA) also called Remote Alarm Indication (RAI) and idle data link sequence in m-bits is mandatory. The other use of m-bits is optional, but when a function other than mandatory functions is used, the specification described here should be applied to guarantee the interworking of terminals. NOTE Some functions, such as PRM, are mandatory in some national standard Priority operations messages Priority messages are transmitted as continuous repetitions of the designated sequence for the duration of the condition initiating the message. It is permissible to interrupt the continuous repetition of the sequence for an interval not to exceed ms not more then once per second, in order to send one or more other maintenance messages. Two priority messages have been defined: After an LFA condition is detected at local end A, the LFA sequence is transmitted to remote end B for the duration of the LFA condition, but not less than one second; a loopback control signal that is used in those applications requiring a continuous enabling signal during operations in a looped condition. Recommendation G.74 (7/95) Superseded by a more recent version 3

8 TABLE 2/G.74 Assigned operations data link messages Priority Messages Function Codeword LFA (also called RAI) Loopback Retention Other Operation Messages Category Function Codeword Loopbacks: Customer Installation Type A Operate Customer Installation Type A Release Customer Installation Type B Operate Customer Installation Type C Operate Payload Operate Payload Release Network Type A Operate Universal Release Protection switching: Operate Line Operate Line 2 Operate Line 3 Operate Line 4 Operate Line 5 Operate Line 6 Operate Line 7 Operate Line 8 Operate Line 9 Operate Line Operate Line Operate Line 2 Operate Line 3 Operate Line 4 Operate Line 5 Operate Line 6 Operate Line 7 Operate Line 8 Operate Line 9 Operate Line 2 Operate Line 2 Operate Line 22 Operate Line 23 Operate Line 24 Operate Line 25 Operate Line 26 Operate Line 27 Acknowledge protection switching action Release protection switch Synchronization: Reserved for further study Reserved for further study Reserved for further study Reserved for further study NOTE Unassigned messages are for further study. 4 Recommendation G.74 (7/95) Superseded by a more recent version

9 Other maintenance or operations messages Other maintenance and operations messages are defined in three general categories of loopbacks, protection switching, and synchronization: Four types of line loopbacks (in which the entire signal including F-bits is returned to the direction sending the initiation signal) are recognized, three on customer premises and one within the network. Each has a defined sequence to operate. One has a unique sequence to release, while all share a common release sequence. A payload loopback (in which only the information bits in a frame are returned) is defined for implementation in a primary hierarchical level terminal. The F-bits in the return direction are generated by the equipment performing loopback. Unique operate and release sequences are defined. The payload loopback also responds to the universal release sequence. For protection switching, 27 sequences are defined to activate protection switching, one sequence is defined to release a protection switch, and one sequence is defined to acknowledge a protection switching action. Activation sequences are of the form P P 2 P 3 P 4 P 5, where the bit sequence P P 2 P 3 P 4 P 5 is the binary representation of the decimal line number x of the line to be switched to the protection line. In these sequences, P is the least significant bit of the binary representation. Four messages are reserved for synchronization-related operations. The details are for further study Performance report from a primary hierarchical level terminal Performance verification is based on the calculation of CRC check sums and comparison with those received in bits e through e 6 as described in These counts, and counts of other events available at the receiving terminal are collected for contiguous one second periods, summarized and formatted into a performance report message that is returned once each second to the originating terminal in the data link for the opposite direction of transmission. The overall length of the Q.92 frame, including opening and closing flags is 5 bytes. Data on the four most recent seconds is structured into an eight-octet information field as shown in Figure. At the end of each one second accumulation interval, a modulo 4 counter is incremented and the most recent data is examined to set the performance bits in the t octets (octets 5 and 6 of Figure ). Data from octets 5 and 6 of the previous performance report are moved to octets 7 and 8 of the current report; previous octets 7 and 8 get moved to current octets 9 and ; previous octets 9 and get moved to current octets and 2, while the previous octets and 2 are discarded. NOTE The SAPI value 4 (decimal) is reserved in the 4 kbit/s data link for use in the performance report. The C/R is set to if the terminal originating the report is within the network; the bit is set to if the report originates from within a customer installation. The TEI is set to all zeros. The Extended Address (EA) is always set to in the octet containing the SAPI, and set to in the octet containing the TEI. The specific values of C/R and TEI defined here are used at the Network Node Interfaces (NNI). Other specific values may be used at local access portion and User-Network Interface (UNI) (see Recommendations G.963 and I.43). NOTE 2 The events tracked for the performance report and their definitions are: CRC error event The occurrence of a received set of check bits that differ from the locally generated code. Severely errored framing event The occurrence of two or more errors in the multiframe alignment sequence within a 3 ms period. Contiguous 3 ms periods shall be examined. Frame synchronization bit error event The occurrence of an error in the multiframe alignment signal. Line code violation The occurrence of a bipolar violation. Controlled slip event The occurrence of a controlled frame slip. Active payload loopback The existence of an operative payload loopback in the terminal originating the performance report. Recommendation G.74 (7/95) Superseded by a more recent version 5

10 Octet No. Octet label Octet content Flag 2 SAPI C/R EA or 3 TEI EA 4 Control 5 6 G3 LV G4 U U2 G5 SL G6 FE SE LB G R G2 Nm N t G3 LV G4 U U2 G5 SL G6 FE SE LB G R G2 Nm N G3 LV G4 U U2 G5 SL G6 FE SE LB G R G2 Nm N t - t -2 One second report 2 G3 LV G4 U U2 G5 SL G6 FE SE LB G R G2 Nm N t FCS Flag T522-95/da Variable Address Control One second report G = G2 = G3 = G4 = G5 = G6 = SE = FE = LV = SL = LB = U, U2 = R = NmN =,,, FCS Variable Interpretation SAPI = 4, C/R = (CI) EA = SAPI = 4, C/R = (Carrier) EA = TEI =, EA = Interpretation Unacknowledged Information Transfer Interpretation CRC Error Event = < CRC Error Event 5 5 < CRC Error Event < CRC Error Event < CRC Error Event 39 CRC Error Event 32 Severely-Errored Framing Event (FE shall = ) Frame Synchronization Bit Error Event (SE shall = ) Line Code Violation Event Slip Event Payload Loopback Activated Under Study for Synchronization Reserved (Default value is ) One-second report modulo 4 counter Interpretation CRC6 Frame Check Sequence NOTE Rightmost bit transmitted first. FIGURE /G.74 Performance report structure at NNI FIGURE /G.74...[DA] = 2 CM Method 2: Twelve-frame multiframe Allocation of the F-bit to the frame alignment signal, multiframe alignment signal and signalling is given in Table 3. 6 Recommendation G.74 (7/95) Superseded by a more recent version

11 TABLE 3/G.74 Allocation of F-bits for the 2-frame multiframe Frame number Frame alignment signal Multiframe alignment signal or signalling 2 S 3 4 S NOTE For multiframe structure, see Basic frame structure at 632 kbit/s 2.2. Frame length The number of bits per frame is 789. The frame repetition rate is 8 Hz F-bits The last five bits of a frame are designated as F-bits, and are used for such purposes as frame alignment, performance monitoring and providing a data link Allocation of F-bits Allocation of the F-bits is given in Table 4. TABLE 4/G.74 Allocation of F-bits Frame number Bit number m 2 3 x x x a m 4 e e 2 e 3 e 4 e 5 m Data link bit. a Remote end alarm bit ( state = alarm, state = no alarm). e i CRC-5 check bit (i = to 5). x Spare bits, to be set at state if not used Frame alignment signal The frame and multiframe alignment signal is, and is carried on the F-bits in frames and 2, excluding bit 789 of frame. Recommendation G.74 (7/95) Superseded by a more recent version 7

12 Cyclic redundancy check Superseded by a more recent version The Cyclic Redundancy Check 5 (CRC-5) Message Block (CMB) is a sequence of 35 serial bits which starts at bit number of frame number and ends at bit number 784 of frame number 4. The CRC-5 message block check bits e, e 2, e 3, e 4 and e 5 occupy the last five bits of the multiframe as shown in Table 4. The check-bit sequence e through e 5 transmitted in multiframe N is the remainder after multiplication by x 5 and then division (modulo 2) by the generator polynomial x 5 + x 4 + x 2 + of the polynomial corresponding to CMB N. The first check bit (e ) is the most significant bit of the remainder; the last check bit (e 5 ) is the least significant bit of the remainder. Each multiframe contains the CRC-5 check bits generated for the corresponding CMB. At the receiver the incoming sequence of 356 serial bits (i.e. 35 bits of CMB and 5 CRC bits), when divided by the generator polynomials, will result in a remainder of in the absence of transmission errors kbit/s data link The bit m shown in Table 4 is used as a data link bit. These bits provide 4 kbit/s data transmission capability associated with the 632 kbit/s digital path Remote end alarm indication After a loss of frame alignment condition is detected at local end A, remote end alarm signal bit a, shown in Table 4, will be transmitted to remote end B. 2.3 Basic frame structure at 248 kbit/s 2.3. Frame length 256 bits, numbered to 256. The frame repetition rate is 8 Hz Allocation of bits number to 8 of the frame Allocation of bits number to 8 of the frame is shown in Table 5A Description of the CRC-4 procedure in bit of the frame Special use of bit of the frame Where there is a need to provide additional protection against simulation of the frame alignment signal, and/or where there is a need for an enhanced error monitoring capability, then bit should be used for a cyclic redundancy check-4 (CRC-4) procedure as detailed below. NOTE Equipment incorporating the CRC-4 procedure should be designed to be capable of interworking with equipment which does not incorporate the CRC-4 procedure, that is, an ability to continue to provide service (traffic) between equipments with and without a CRC-4 capability. This can be achieved either manually (e.g. by straps) or automatically. For the manual case, the equipment incorporating the CRC-4 procedure should be capable of fixing bit of the frame to the binary state (see Table 5A, Note ). For the automatic case, this can be achieved at the equipment having the CRC-4 capability either: as a higher-layer function under the control of a network management facility (e.g. a TMN) the details are for further study; or as a lower-layer function using a modified CRC-4 multiframe alignment algorithm as described in Annex B/G The allocation of bits to 8 of the frame is shown in Table 5B for a complete CRC-4 multiframe Each CRC-4 multiframe, which is composed of 6 frames numbered to 5, is divided into two 8-frame Sub- Multiframes (SMF), designated SMF I and SMF II which signifies their respective order of occurrence within the CRC-4 multiframe structure. The SMF is the Cyclic Redundancy Check-4 (CRC-4) block size (i.e. 248 bits). The CRC-4 multiframe structure is not related to the possible use of a multiframe structure in 64 kbit/s channel time slot 6 (see ). 8 Recommendation G.74 (7/95) Superseded by a more recent version

13 TABLE 5A/G.74 Allocation of bits to 8 of the frame Bit number Alternate frames Frame containing the frame alignment signal Frame not containing the frame alignment signal S i (Note ) Frame alignment signal S i A S a4 S a5 S a6 S a7 S a8 (Note ) (Note 2) (Note 3) (Note 4) NOTES S i = Bits reserved for international use. One specific use is described in Other possible uses may be defined at a later stage. If no use is realized, these bits should be fixed at on digital paths crossing an international border. However, they may be used nationally if the digital path does not cross a border. 2 The bit is fixed at to assist in avoiding simulations of the frame alignment signal. 3 A = Remote alarm indication. In undisturbed operation, set to ; in alarm condition, set to. 4 S a4 to S a8 = Additional spare bits whose use may be as follows: i) Bits S a4 to S a8 may be recommended by ITU-T for use in specific point-to-point applications (e.g. transcoder equipments conforming to Recommendation G.76). ii) Bit S a4 may be used as a message-based data link to be recommended by ITU-T for operations, maintenance and performance monitoring. If the data link is accessed at intermediate points with consequent alterations to the S a4 bit, the CRC-4 bits must be updated so as to retain the correct end-to-end path termination functions associated with the CRC-4 procedure (see ). The data-link protocol and messages are for further study. iii) Bits S a5 to S a7 are for national usage where there is no demand on them for specific point-to-point applications [see i) above]. iv) One of the bits S a4 to S a8 may be used in a synchronization interface to convey PDH synchronization status messages, as described in Bits S a4 to S a8 (where these are not used) should be set to on links crossing an international border Use of bit in 248 kbit/s CRC-4 multiframe In those frames containing the frame alignment signal (defined in 2.3.2), bit is used to transmit the CRC-4 bits. There are four CRC-4 bits, designated C, C 2, C 3 and C 4 in each SMF. In those frames not containing the frame alignment signal (see 2.3.2), bit is used to transmit the 6-bit CRC-4 multiframe alignment signal and two CRC-4 error indication bits (E). The CRC-4 multiframe alignment signal has the form. The E-bits should be set to until both basic frame and CRC-4 multiframe alignment are established (see clause 4/G.76). Thereafter, the E-bits should be used to indicate received errored sub-multiframes by setting the binary state of one E-bit from to for each errored sub-multiframe. Any delay between the detection of an errored submultiframe and the setting of the E-bit that indicates the error state must be less than second. NOTES The E-bits will always be taken into account even if the SMF which contains them is found to be errored, since there is little likelihood that the E-bits themselves will be errored. 2 In the short term, there may exist equipments which do not use the E-bits; in this case the E-bits are set to binary. Recommendation G.74 (7/95) Superseded by a more recent version 9

14 TABLE 5B/G.74 CRC-4 multiframe structure Sub-multiframe (SMF) Frame number Bits to 8 of the frame Multiframe I C C 2 C 3 C 4 A A A A S a4 S a4 S a4 S a4 S a5 S a5 S a5 S a5 S a6 S a6 S a6 S a6 S a7 S a7 S a7 S a7 S a8 S a8 S a8 S a8 II C C 2 C 3 E C 4 E A A A A S a4 S a4 S a4 S a4 S a5 S a5 S a5 S a5 S a6 S a6 S a6 S a6 S a7 S a7 S a7 S a7 S a8 S a8 S a8 Sa8 NOTES E = CRC-4 error indication bits (see ). 2 S a4 to S a8 = Spare bits (see Note 4 to Table 5A). 3 C to C 4 = Cyclic Redundancy Check-4 (CRC-4) bits (see and ). 4 A = Remote alarm indication (see Table 5A) Cyclic Redundancy Check (CRC) Multiplication/division process A particular CRC-4 word, located in sub-multiframe N, is the remainder after multiplication by x 4 and then division (modulo 2) by the generator polynomial x 4 + x +, of the polynomial representation of sub-multiframe N. NOTES When representing the contents of the check block as a polynomial, the first bit in the block, i.e. frame, bit or frame 8, bit, should be taken as being the most significant bit. Similarly, C is defined to be the most significant bit of the remainder and C 4 the least significant bit of the remainder. 2 There may be a need to update CRC-4 bits at intermediate equipments which access the S a4 bit message-based datalink (see ) Encoding procedure i) The CRC-4 bits in the SMF are replaced by binary s. ii) The SMF is then acted upon by the multiplication/division process referred to in iii) The remainder resulting from the multiplication/division process is stored, ready for insertion into the respective CRC-4 locations of the next SMF. NOTE The CRC-4 bits thus generated do not affect the result of the multiplication/division process in the next SMF because, as indicated in i) above, the CRC-4 bit positions in an SMF are initially set to during the multiplication/division process Decoding procedure i) A received SMF is acted upon by the multiplication/division process referred to in , after having its CRC-4 bits extracted and replaced by s. ii) The remainder resulting from this division process is then stored and subsequently compared on a bit-bybit basis with the CRC bits received in the next SMF. Recommendation G.74 (7/95) Superseded by a more recent version

15 iii) Superseded by a more recent version If the remainder calculated in the decoder exactly corresponds to the CRC-4 bits received in the next SMF, it is assumed that the checked SMF is error free Updating procedure at intermediate path points in a message-based data-link application The S a4 bit may be used as a message-based data-link within 248 kbit/s paths [see Note 4, ii) to Table 5A]. Situations are envisaged where access to this data link could be required at points on the path between the true path termination points, e.g. reporting of error performance data from intermediate sites along the path. In such situations, it is important that the logical path termination role of CRC-4 is not invalidated or impaired. Hence, any changes to the S a4 bits within a SMF at an intermediate path point does not imply a recalculation of the CRC-4 bits over the whole SMF, but rather their update as a linear recoding function in respect of specific deterministic binary changes of the S a4 bits only. Annex C/G.76 gives further information regarding this updating procedure Synchronization Status: S an One of the S a4 to S a8 bits, (the actual S a bit is for operator selection), is allocated for PDH Synchronization Status Messages. To prevent ambiguities in pattern recognition, it is necessary to align the first bit (S an, Table 5C) with frame (Table 5D) of a G.74 multiframe. Table 5C gives the assignment of bit patterns to the four synchronization levels agreed to within ITU-T. Two additional bit patterns are assigned: one to indicate that quality of the synchronization is unknown and the other to indicate that the signal should not be used for synchronization. The remaining codes are reserved for quality levels defined by individual operators. Table 5D gives the numbering of the S an (n = 4, 5, 6, 7, 8) bits. A S an bit is organized as a 4 bit nibble S an to S an4. S an is the most significant bit, S an4 is the least significant bit. NOTE The message set in San to San4 is a copy of the set defined in SDH bits 5 to 8 of byte S. 2.4 Basic frame structure at 8448 kbit/s 2.4. Frame length The number of bits per frame is 56. They are numbered from to 56. The frame repetition rate is 8 Hz Frame alignment signal The frame alignment signal is and occupies the bit-positions to 8 and 529 to Service digits Bit 535 is used to convey alarm indication (bit 535 at state = alarm; bits 535 at state = no alarm). Bit 536 is left free for national use and should be fixed at on paths crossing the international border. The same applies to bits 9-4 in the case of channel-associated signalling. 2.5 Basic frame structure at kbit/s 2.5. Multiframe length The number of bits per multiframe is 476 bits Multiframe overhead bits The multiframe are divided into seven M-subframes each with 68 bits; each M-subframe is further divided into 8 blocks of 85 bits: bit for overhead and 84 bits for payload (see Figure 2). Thus, there are 56 overhead bits per multiframe Allocation of the multiframe overhead bits The overhead bits are the first bit of the eight 85-bit blocks in each of the seven M-subframes in a multiframe, as shown in Figure 2. The 56 overhead bits are: 2 X-bits, 2 P-bits, 3 M-bits, 28 F-bits, and 2 C-bits. Recommendation G.74 (7/95) Superseded by a more recent version

16 TABLE 5C/G.74 Synchronization Status Message (SSM) bit allocation QL S an, S an2, S an3, S an4 (Note) Synchronization Quality Level (QL) description Quality unknown (existing synchronization network) Reserved 2 Rec. G.8 3 Reserved 4 G.82 transit 5 Reserved 6 Reserved 7 Reserved 8 G.82 local 9 Reserved Reserved Synchronous Equipment Timing Source (SETS) 2 Reserved 3 Reserved 4 Reserved 5 Do not use for synchronization NOTE n = 4, 5, 6, 7 or 8 (i.e. one S a bit only) depending on operator selection X-bits (X, X2) X and X2 are used to indicate received errored multiframes to the remote-end (remote alarm indication RAI or yellow signal); these bits are set to binary (i.e., X = X2 = ) during error free condition, and to binary (i.e. X = X2 = ) if Loss of Signal (LOS), Out of Frame (OOF), Alarm Indication Signal (AIS), or Slips are detected in the incoming signal. The maximum allowed rate of change of state for the X-Bits is once a second; therefore, the X-Bits should be set to binary for a length of time equal to the length of the error condition, but rounded-up to the next integer P-bits (P, P2) P and P2 are used for performance monitoring; these bits carry parity information calculated over the 474 payload bits in the preceding multiframe: P = P2 = if the digital sum of all payload bits is one, and P = P2 = if the digital sum of all payload bits is zero. The P-bits are calculated and may be modified at each section of a facility; therefore, the P-bits provide SECTION performance information NOT end-to-end performance information Multiframe alignment signal (M, M2, M3) The multiframe alignment signal (M =, M2 =, M3 = ) is used to locate all seven M-subframes, within the multiframe. 2 Recommendation G.74 (7/95) Superseded by a more recent version

17 TABLE 5D/G.74 S an numbering in TimeSlot (TS) for use in synchronization status message Frame number Bit Bit 2 Bit 3 Bit 4 Bit 5 Bit 6 Bit 7 Bit 8 C A S a4 S a5 S a6 S a7 S a8 S 2 C 2 M 3 A S a42 S a52 S a62 S a72 S a82 F 4 C 3 I 5 A S a43 S a53 S a63 S a73 S a83 6 C 4 7 A S a44 S a54 S a64 S a74 S a84 8 C 9 A S a4 S a5 S a6 S a7 S a8 S C 2 M A S a42 S a52 S a62 S a72 S a82 F 2 C 3 II 3 E A S a43 S a53 S a63 S a73 S a83 4 C 4 5 E A S a44 S a54 S a64 S a74 S a M-subframe alignment signal (F, F2, F3, F4) The M-subframe alignment signal (F =, F2 =, F3 =, F4 = ) is used to identify the overhead bit positions C-bits (C, C2, C3, C2,... C ij,... C73) In general kbit/s signals could be: a) unchannelized for bulk data transport; and b) channelized for multiplex applications. In either case, the C ij bit positions are available for specific uses, and must be settable by kbit/s sources. The way that these C ij bits are used determine the features available in the kbit/s signal, through the embedded operations channels: The kbit/s multiplexing application (M23), uses the C-bits to indicate justification (Recommendation G.752). This standard describes the C-bit Parity application which does not use stuffing bits for justification. Recommendation G.74 (7/95) Superseded by a more recent version 3

18 Multiframe (476 bits) X X2 P P2 M M2 M3 bits bits bits bits bits bits 679 bits First M-subframe (68 bits) X F C F2 C2 F3 C3 info info info info info info info F4 84 info The 56 overhead bits sequential positions as follows: X F C F2 C2 F3 C3 F4 X2 F C2 F2 C22 F3 C23 F4 P F C3 F2 C32 F3 C33 F4 P2 F C4 F2 C42 F3 C43 F4 M F C5 F2 C52 F3 C53 F4 M2 F C6 F2 C62 F3 C63 F4 M3 F C7 F2 C72 F3 C73 F4 T /d FIGURE 2/G kbit/s multiframe structure FIGURE 2/G.74...[DB] = 2 CM Both, the unchannelized application as well as the channelized C-bit Parity multiplex application ) use the C-bits as described in Allocation of C-bits for C-bit Parity application Regardless of the application (unchannelized or channelized) the C-bits for C-bit Parity application are allocated as follows: C: Application Identification Channel (AIC) For C-bits Parity application this bit shall be set to binary. C2: Network Requirements (N r ) Reserved for future network use. It shall be set to binary. C3: Far-End Alarm and Control (FEAC) bit is used for two purposes: ) Alarm signals to send alarm or status information from the far-end terminal back to the near-end terminal; and ) The C-Bit Parity multiplex application for channelized signals uses a two-step multiplexing process to multiplex primary rate signals (544 or 248 kbit/s) to the kbit/s level. In the first step, four 544 kbit/s or three 248 kbit/s lines are multiplexed together to form an integral signal at a bit rate f e, (pseudo-632 kbit/s level). In the second step, seven pseudo-632 kbit/s level, each at a bit rate f e are multiplexed together to form a kbit/s signal with enhanced operations features. The bit rate f e (nominally ) kbit/s) is chosen such that when the seven pseudo-632 kbit/s level signals are combined, along with full time kbit/s level justification and the 56 frame overhead bits, the resultant output bit rate will nominally be kbit/s. This multiplexing process is the same as that defined for the M23 application except that in the C-bit Parity case all seven intermediate timeslots, one in each of the seven M-subframes, are justified at every justification opportunity. Since justification occurs % of the time, the C-bits are not needed to denote justification, and they can be used for other purposes. 4 Recommendation G.74 (7/95) Superseded by a more recent version

19 2) Control signals to initiate kbit/s and 544 kbit/s line loopbacks at the far-end terminal, from the near-end terminal. At international interfaces, initiation of control loopback signal is optional and the application of this functionality should be at the discretion of the respective Administrations. The FEAC signal consists of a repeating 6-bit codeword with a general format of xxx xxx, rightmost bit transmitted first (where x can be a or a ). To report alarm/status conditions, the 6-bit codeword must be repeated at least times, or while the condition exists, whichever is longer. (Table 6 shows the alarms/status codewords assigned). These codewords shall be transmitted only after the event has been declared: for example, a kbit/s AIS defect would be detected and then timed for several seconds before declaring AIS failure, at which time the appropriate codeword would be transmitted. To send loopback control commands, two codewords must be sent: the first one repeated ten times to activate/ de-activate, the other also repeated ten times to specify the line number, therefore, each loopback command consists of 2 6-bit codewords. (Table 7 shows the control codewords assigned.) Control words take precedence over alarm signals. When no alarm/status or control is being transmitted, the FEAC bits must be all set to binary. C2, C22, C23 Not used; must be set to binary. C3, C32, C33 CP-bits are used to carry path (end-to-end facility) parity information. The network terminating equipment (NTE) that originates the kbit/s signal must set these bits (C3 = C32 = C33) to the same value as the P-bits. The CP-bits must not be modified along the kbit/s facility path. C4, C42, C43 FEBE-bits are used to carry far-end block error information. All three FEBE bits are set to binary (C4 = C42 = C43 = ) if no errors are detected in the M-bits, or F-bits, or indicated by the CP-bits. If any error condition (errored M-bits, errored F-bits, or parity in CP-bits) is detected within the multiframe, the FEBE bits must be set to any combination of s or s (except ). C5, C52, C53 DL t bits are used for a 28.2 kbit/s-terminal-to-terminal path maintenance data link. The implementation of this data link is optional but if implemented, it shall conform to the rules set forth in this subclause. The messages carried in the path maintenance data link utilize the frame structure, field definitions, and elements of procedure of the LAPD protocol defined in Recommendation Q.92 but with different addresses. The structure of the LAPD message-oriented signals is defined in Table 8. Table 9 shows the contents and structure of the information field for each of the four message types defined: Common Language Path ID, ITU-T Path ID, Test ID, and Idle Signal ID. The information field contains 6 data elements to identify: ) the test type; 2) the equipment type; 3) the central office location; 4) the frame (within the central office); 5) the unit (within the frame); and 6) information specific to the test type. These signals shall be transmitted continuously at a minimum rate of once per second. When LAPD messages are not being transmitted (i.e. the data link is idle), LAPD flags () shall be continuously transmitted. If terminal-to-terminal data link function is not implemented, all three bits shall be set to binary (C5 = C52 = C53 = ). Other applications for the path maintenance data link are for further study. C6, C62, C63 Not used; must be set to binary. C7, C72, C73 Not used; must be set to binary. Recommendation G.74 (7/95) Superseded by a more recent version 5

20 TABLE 6/G.74 FEAC alarm/status codewords FEAC alarm/status codewords Alarm/status condition Codeword Out of frame at kbit/s Equipment failure at 544 or 248 kbit/s (NSA) Equipment failure at 544 or 248 kbit/s (SA) LOS/HBER at kbit/s Equipment failure at kbit/s (NSA) Multiple LOS/HBER at 544 or 248 kbit/s AIS received kbit/s Equipment failure for kbit/s (SA) Idle received at kbit/s Common equipment (NSA) Single LOS/HBER at 544 or 248 kbit/s NOTES The rightmost bit of each codeword is transmitted first. 2 SA denotes service affecting equipment failure forcing out-of-service state, indicating a defect requiring immediate attention. 3 NSA denotes non-service affecting equipment failure indicating a defect in equipment that is not activated, not available, or suspended; it requires attention, but not high priority Special patterns used at kbit/s Two special patterns are defined for the kbit/s signals independently of how the C-bits are used: AIS and IDLE, as described next Alarm Indication Signal (AIS) The AIS is a signal with valid multiframe and M-subframe alignment signals, and valid P-bits. The information bits are set to a... sequence, starting with a binary one () after each M-bit, F-bit, X-bit, P-bit, and C-bit. The C-bits are set to binary zero (C =, C2 =, C3 = ). The X-bits are set to binary one (X =, X2 = ) Idle Signal (Idle) The Idle Signal is a signal with valid multiframe and M-subframe alignment signals, and valid P-bits. The information bits are set to a... sequence, starting with a binary one () after each M-bit, F-bit, X-bit, and C-bit. The C-bits are set to binary zero (C =, C2 =, C3 = ), in the third M-subframe (C3, C32, C33); the remaining C-Bits (three C-bits in M-subframes, 2, 4, 5, 6, and 7) may be individually set to one or zero, and may vary with time. The X-bits are set to binary one (X =, X2 = ). 6 Recommendation G.74 (7/95) Superseded by a more recent version

21 TABLE 7/G.74 FEAC control codeword FEAC control codewords Command Codeword Activate loopback De-activate loopback kbit/s line All 544 or 248 kbit/s lines 544 or 248 kbit/s Line No., Group # 544 or 248 kbit/s Line No. 2, Group # 544 or 248 kbit/s Line No. 3, Group # 544 kbit/s Line No. 4, Group # 544 or 248 kbit/s Line No., Group #2 544 or 248 kbit/s Line No. 2, Group #2 544 or 248 kbit/s Line No. 3, Group #2 544 kbit/s Line No. 4, Group #2 544 or 248 kbit/s Line No., Group #3 544 or 248 kbit/s Line No. 2, Group #3 544 or 248 kbit/s Line No. 3, Group #3 544 kbit/s Line No. 4, Group #3 544 or 248 kbit/s Line No., Group #4 544 or 248 kbit/s Line No. 2, Group #4 544 or 248 kbit/s Line No. 3, Group #4 544 kbit/s Line No. 4, Group #4 544 or 248 kbit/s Line No., Group #5 544 or 248 kbit/s Line No. 2, Group #5 544 or 248 kbit/s Line No. 3, Group #5 544 kbit/s Line No. 4, Group #5 544 or 248 kbit/s Line No., Group #6 544 or 248 kbit/s Line No. 2, Group #6 544 or 248 kbit/s Line No. 3, Group #6 544 kbit/s Line No. 4, Group #6 544 or 248 kbit/s Line No., Group #7 544 or 248 kbit/s Line No. 2, Group #7 544 or 248 kbit/s Line No. 3, Group #7 544 Line No. 4, Group #7 NOTES The commands that refer to 544 or 248 kbit/s line apply only to a channelized C-bit Parity applications. 2 Group refers to the four 544 kbit/s or three 248 kbit/s signals that form the intermediate internal f e signal (see footnote ); seven of these groups (plus justification) are combined to form the kbit/s signal. 3 The rightmost bit of each codeword is transmitted first. 4 To activate or de-activate loopback, the appropriate activate or de-activate 6-bit codeword is transmitted ten times followed immediately by ten repetitions of the 6-bit codeword corresponding to the required line number. Thus, the total length of loopback control message is 2 6-bit words. Recommendation G.74 (7/95) Superseded by a more recent version 7

22 TABLE 8/G.74 LAPD message structure Octet no. Octet label Octet content Flag 2 2 SAPI CR EA 2 or 2 3 TEI EA 2 4 Control 2 Path Identifier (CL or ITU-T), Information field Idle Signal Id, or Test Signal Id. (see Table 9) N FCS See below N FCS Flag Interpretation 2 Response Message SAPI CR EA 2 2 Interpretation SAPI = 5, C/R = (DTE), EA = SAPI = 5, C/R = (carrier), EA = TEI/EA Interpretation 2 TEI =, EA = Control Interpretation 2 Fixed value: Unacknowledged Information Transfer Information field variable FCS Frame Check Sequence Interpretation See Table 9 Interpretation CRC-6 Frame Check Sequence, 6-bit code NOTE The source of the identification messages shall generate the FCS and the zero stuffing required for transparency. Zero stuffing by a transmitter prevents the occurrence of the flag patten () in the bits between the opening and closing flags of a frame, by inserting a zero after any sequence of five consecutive ones. The receiver removes a zero following five consecutive ones. 3 Characteristics of frame structure carrying channels at various bit rates in 544 kbit/s 3. Interface at 544 kbit/s carrying 64 kbit/s channels 3.. Frame structure 3... Number of bits per 64 kbit/s channel time slot Eight, numbered to 8. 8 Recommendation G.74 (7/95) Superseded by a more recent version

23 TABLE 9/G.74 Information field contents for data-link messages CL path identification ITU-T path identification Data elements Binary value Data elements Binary value Type ( octet) LIC xxxx xxxx... ( octets) FIC xxxx xxxx... ( octets) EIC xxxx xxxx... ( octets) Unit xxxx xxxx... (6 octets) CL-path ID Type ( octet) Equipment ID LIC xxxx xxxx... ( octets) Location ID FIC xxxx xxxx... ( octets) Frame ID EIC xxxx xxxx... ( octets) Unit ID Unit xxxx xxxx... (6 octets) ITU-T path ID Equipment ID Location ID Frame ID Unit ID CL-facility ID xxxx xxxx (38 octets) CL-facility ID ITU-T facility ID xxxx xxxx (44 octets) ITU-facility ID Idle signal identification Test signal identification Data elements Binary value Data elements Binary value Type ( octet) LIC xxxx xxxx... ( octets) FIC xxxx xxxx... ( octets) EIC xxxx xxxx... ( octets) Unit xxxx xxxx... (6 octets) Idle signal ID Type ( octet) Equipment ID LIC xxxx xxxx... ( octets) Location ID FIC xxxx xxxx... ( octets) Frame ID EIC xxxx xxxx... ( octets) Unit ID Unit xxxx xxxx... (6 octets) Test signal ID Equipment ID Location ID Frame ID Unit ID Port no. xxxx xxxx (38 octets) Port no. Gen. no. xxxx xxxx (38 octets) Generator no. Location Frame ID Unit ID CL-facility ID ITU-T-facility ID Port no. Generator no. Uniquely identifies the city and building where the equipment is located. Uniquely identifies the floor, aisle and bay (within the building) where the equipment is located. Uniquely identifies the shelf and slot (within the frame) where the card (which generates the signal) is located. Identifies a specific kbit/s path, using Common Language conventions and codes. Identifies a specific kbit/s path, using Recommendation M.4 conventions and codes for facility designation. Identifies the equipment port number that initiates the idle signal. Identifies the equipment generator number that initiates the test signal. NOTE The ASCII null character shall be used to indicate the end of the string when the full length of the data field is not needed for a given element. The remaining bit positions of the data element may contain ones, zeros, or any combination of ones and zeros. In those cases where a data element is not needed for a given message, the first octet of the data element shall contain the ASCII null character, the remaining bit positions may contain ones, zeros, or any combination of ones and zeros. Recommendation G.74 (7/95) Superseded by a more recent version 9

24 3...2 Number of 64 kbit/s channel time slots per frame Bits 2 to 93 in the basic frame carry 24 octet interleaved 64 kbit/s channel time slots, numbered to Allocation of F-bit Refer to Use of 64 kbit/s channel time slots Each 64 kbit/s channel time slot can accommodate, for example, a PCM-encoded voiceband signal conforming to Recommendation G.7 or data information with a bit rate up to 64 kbit/s Signalling Two alternative methods as given in and are recommended Common channel signalling One 64 kbit/s channel time slot is used to provide common channel signalling at a rate of 64 kbit/s. In the case of the 2-frame multiframe method of , the pattern of the S-bit may be arranged to carry common channel signalling at a rate of 4 kbit/s or a sub-multiple of this rate Channel associated signalling Allocation of signalling bits for the 24-frame multiframe As can be seen in Table, there are four different signalling bits (A, B, C and D) in the multiframe. This channel associated signalling can provide four independent 333-bit/s signalling channels designated A, B, C and D, two independent 667-bit/s signalling channels designated A and B (see Note) or one 333-bit/s signalling channel. NOTE When only four state signalling is required, the A, B signalling bits previously associated with frames 6 and 2 respectively should be mapped into the A, B, C, D signalling bits of frames 6, 2, 8 and 24 respectively as follows: A = A, B = B, C = A, D = B. In this case, the ABCD signalling is the same as the AB signalling specified in Allocation of signalling bits for the 2-frame multiframe Based on agreement between the Administrations involved, channel-associated signalling is provided for intra-regional circuits according to the following arrangement. A multiframe comprises 2 frames as shown in Table. The multiframe alignment signal is carried on the S-bit as shown in the table. Frames 6 and 2 are designated as signalling frames. Bit eight in each channel time slot is used in every signalling frame to carry the signalling associated with that channel. 3.2 Interface at 544 kbit/s carrying 32 kbit/s channel time slots (see Note) NOTE This interface provides for the carrying of 32 kbit/s information. The interface will be used between network nodes and will apply to primary rate multiplexing equipment, digital cross-connect equipment, transcoder and other equipment relevant to the network nodes. Switching in this case is assumed to take place on a 64 kbit/s basis Frame structure Number of bits per 32 kbit/s channel time slot Four, numbered to Number of 32 kbit/s channel time slots per frame Bits 2 to 93 in the basic frame can carry forty-eight 4-bit interleaved 32 kbit/s channel time slots, numbered to Allocation of F-bits Refer to Recommendation G.74 (7/95) Superseded by a more recent version